1. Field of the Invention
The invention relates to the field of 3-phase rectifiers, active power filters, and grid-connected inverters.
2. Description of the Prior Art
The invention in this document covers vast applications spanning from power factor corrected rectifiers in front of, and active power filters in parallel with, electronic equipment such as computers, communication, motion control, aviation, space electronics, and grid-connected inverters for distributed power generation.
Power Factor Corrected Rectifiers
In recent years, the usage of modern electronics equipment has been widely proliferating. The electronics equipment usually have a rectifier of single-phase or three-phases in the front end. Three-phases are more desirable for high power applications. A three-phase rectifier is a device that converts three-phase sinusoidal ac power into dc power. Traditional rectifiers draw pulsed current from the ac main as shown in FIG. 1, which causes significant harmonic pollution, low power factor, reduced transmission efficiency, harmful electromagnetic interference to neighborhood appliances, as well as overheating of transformers. In order to solve these problems, many international agencies have proposed harmonic restrictions to electronic equipment. As a result, a vast number of power factor corrected (PFC) rectifers have been proposed to comply with these regulations.
A three-phase power factor corrected rectifier is a device that converts three-phase sinusoidal ac power into dc power while the input currents are sinusoidal and unity power factor, as shown in FIG. 2. Many three-phase topologies are suitable for implementing PFC function for rectification. Usually, high frequency active switches are used in the rectifiers to realize the PFC function. The control methods that modulate the pulse width of the switches are an important issue in the power electronics research. A third harmonic injection method was reported for a dual-boost converter with center-tapped dc-link and split output capacitors. This method achieves low current distortion. However, it is not convenient to generate the third harmonic signal tuned to the right frequency and right amplitude. Hysteresis control and d-q transformation control were frequently used control approaches. Hysteresis control results in variable switching frequency that is difficult for EMI filter design. The d-q approach is based on digital implementation that leads to complicated systems. An analog control method with constant switching frequency modulation was reported for a particular rectifier, where several multipliers are necessary to implement the three phase current references. Due to the disadvantages of variable frequency or complexity of implementation, three-phase PFC rectifiers are not yet commercially available.
Active Power Filters
One alternative to deal with the current harmonics generated by traditional rectifiers is to use active power filters (APF). Considering the electronic equipment with traditional rectifier as nonlinear loads to the ac main, a three-phase APF is a device that is connected in parallel to and cancels the reactive and harmonic currents from one or a group of nonlinear loads so that the resulting total current drawn from the ac main is sinusoidal as shown in FIG. 3. By comparison, it will be noted that where a PFC unit is usually inserted in the energy pass, which processes all the power and corrects the current to unity power factor, APF provides only the harmonic and reactive power to cancel the one generated by the nonlinear loads. In this case, only a small portion of the energy is processed, which may result in overall higher energy efficiency and higher power processing capability. Most APF control methods proposed previously need to sense the three-phase line voltages and the three-phase nonlinear load currents, and then manipulate the information from these sensors to generate three-phase current references for the APF. Since the reference currents have to reflect the load power of the nonlinear load, several multipliers are needed to scale the magnitude of the current references. A control loop is necessary to control the inverter to generate the reactive and harmonic current required by the nonlinear load. These functions are generally realized by a digital signal processing (DSP) chip with fast analog-to-digital (A/D) converters and high-speed calculations. The complex circuitry which results in high cost and unreliable systems, often prevents this technique from practical applications. Some approaches that sense the mains line current were reported for single-phase APF and for three-phase APF. The overall circuitry is reduced. However, multipliers, input voltage sensors are still necessary. High speed DSPs are still used in three-phase systems due to the complexity of the systems.
Grid-connected Inverters
Distributed power generation is the trend in the future in order to promote new power generation technologies and reduce transmition costs. An effective use of natural resources and renewable resources as alternatives of fossil and nuclear energy in generation of electricity has the effect of protecting the environment. In order for the alternative energy sources to impact the energy supply in the future, they need to be connected to the utility grid. Therefore, grid-connected inverters are the key elements for the distributed power generation systems.
A grid-connected inverter is a device that converts dc power to ac power of single phase or three-phases that is injected to the utility grid. In order for an alternative energy source to be qualified as a supplier, sinusoidal current injection is required as shown in FIG. 4. Again, control methods are cruicial. In the past, d-q transformer modulation based on digital implementation was often employed for a standard six-switched bridge inverter topology. The complexity results in low reliability and high cost. In addition, short-through hazard exists in this inverter.